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What is a Circuit?

Wall's Corners

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Title: What is a Circuit?
Author: Wall's Corners
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Overview Welcome to circuits 101! One of the first things you’ll encounter when learning about electronics is the concept of a circuit . T...

<div class="section tutorial-page">
<h2 id="overview">Overview</h2>
<p>Welcome to circuits 101! One of the first things you’ll encounter when learning about electronics is the concept of a <strong>circuit</strong>. This tutorial will explain what a circuit is, as well as discuss <strong>voltage</strong> in further detail.</p>
<p><a href="http://bit.ly/2odK90Q" target="_blank"><img alt="Picture of Circuit" src="http://bit.ly/2pcMYUl" /></a></p>
<p><em>A simple circuit, involving a <a href="http://bit.ly/2odR1ew" target="_blank">button</a>, an <a href="http://bit.ly/1WBzUB8" target="_blank">LED</a>, and a <a href="http://bit.ly/2odGTTm" target="_blank">resistor</a>, built two different ways.</em></p>
<h3>Suggested Reading</h3>
<p>There are a few concepts you should have a good understanding of to help you get the most out of this tutorial.</p>
<ul>
<li><a href="http://bit.ly/2pcRZMP" target="_blank">What is Electricity?</a></li>
<li><a href="http://bit.ly/2pfbeCs" target="_blank">Electric Power</a></li>
<li><a href="http://bit.ly/2nL8cZL" target="_blank">Voltage, Current, Resistance, and Ohm’s Law</a></li>
<li><a href="http://bit.ly/2pfeT39" target="_blank">Alternating Current (AC) vs Direct Current (DC)</a></li>
<li><a href="http://bit.ly/2nLfywj" target="_blank">Polarity</a></li>
</ul>
</div>
<div class="section tutorial-page">
<h2 id="circuit-basics">Circuit Basics</h2>
<h3>Voltage and How it Works</h3>
<p>You’ve probably heard that a battery or a wall outlet has a certain number of <strong>volts</strong>. This is a measurement of the electrical <strong>potential</strong> produced by the battery, or the utility grid connected to the wall outlet.</p>
<p>All those volts are sitting there waiting for you to use them, but there’s a catch: <strong>in order for electricity to do any work, it needs to be able to move</strong>. It’s kind of like a blown-up balloon; if you pinch it off, there is air in there that <em>could</em> do something if it’s released, but it won’t actually do anything until you let it out.</p>
<p>Unlike air coming out of a balloon, electricity can only flow through materials that can conduct electricity, such as copper wire. If you connect a wire to a battery or wall outlet (<strong>WARNING:</strong> the voltage in a wall outlet is dangerous, don’t do this!), you will be giving the electricity a path to follow. But if the wire isn’t connected to anything else, the electricity won’t have anywhere to go and still won’t move.</p>
<p><a href="http://bit.ly/2pfdn1j" target="_blank"><img alt="alt text" src="http://bit.ly/2pfdn1j" /></a></p>
<p>What makes electricity move? <strong>Electricity wants to flow from a higher voltage to a lower voltage.</strong> This is exactly like the balloon: the pressurized air in the balloon wants to flow from inside the balloon (higher pressure) to outside the balloon (lower pressure). If you create a conductive path between a higher voltage and a lower voltage, electricity will flow along that path. And if you insert something useful into that path like an <a href="http://bit.ly/1UhB2tm" itemprop="url" target="_bank" title="LED"><strong>LED</strong></a>, the flowing electricity will do some work for you, like lighting up that LED. Huzzah!</p>
<p><a href="http://bit.ly/2pfgmXi" target="_blank"><img alt="alt text" src="http://bit.ly/2pfgmXi" /></a></p>
<p>So, where do you find a higher voltage and a lower voltage? Here’s something really useful to know: <strong>every source of electricity has two sides</strong>. You can see this on batteries, which have metal caps on both ends, or your wall outlet that has two (or more) holes. In batteries and other <a href="http://bit.ly/2nL12EF" target="_blank">DC (Direct Current)</a> voltage sources, these sides (often called <strong>terminals</strong>) are named <strong>positive</strong> (or “+”), and <strong>negative</strong> (or “-”).</p>
<p>Why does every source of electricity have two sides? This goes back to the idea of “potential”, and that you need a voltage difference in order to get electricity to flow. It sounds silly, but you can’t have a difference without two things to be different. In any power supply, the positive side will have a higher voltage than the negative side, which is exactly what we want. In fact, when we measure voltage, we usually say that the negative side is 0 volts, and the positive side is however many volts the supply can provide.</p>
<p>Electrical sources are like pumps. Pumps always have two sides, an outlet that blows something out, and an inlet that sucks something in. Batteries and generators and solar panels work the same way. Something inside them is hard at work moving electricity towards the outlet (the positive side), but all that electricity leaving the device creates a void, which means that the negative side needs to pull electricity in to replace it.<a href="http://bit.ly/2pf61KS" target="_blank">*</a></p>
<p>What have we learned so far?</p>
<ul>
<li>Voltage is potential, but electricity needs to flow to do anything useful.</li>
<li>Electricity needs a path to flow through, which must be an electrical conductor such as copper wire.</li>
<li>Electricity will flow from a higher voltage to a lower voltage.</li>
<li>DC voltage sources always have two sides, called positive and negative, with the positive side a higher voltage than the negative side.</li>
</ul>
<h3>The Simplest Circuit</h3>
<p>We’re finally ready to make electricity work for us! If we connect the positive side of a voltage source, through something that does some work such as a <a href="http://bit.ly/1Pt3Au6" itemprop="url" target="_bank" title="Light"><strong>Light</strong></a> Emitting Diode (LED), and back to the negative side of the voltage source; electricity, or <strong>current</strong>, will flow. And we can put things in the path that do useful things when current flows through them, like LEDs that <a href="http://bit.ly/1T1Ll2n" itemprop="url" target="_bank" title="Create your own musical light show with Raspberry Pi"><strong>light</strong></a> up.</p>
<p><a href="http://bit.ly/2nKYAOC" target="_blank"><img alt="alt text" src="http://bit.ly/2nKYAOC" /></a></p>
<p><strong>This circular path, which is always required to get electricity to flow and do something useful, is called a circuit.</strong> A circuit is a path that starts and stops at the same place, which is exactly what we’re doing.</p>
<p><a href="http://bit.ly/2pflN8N" target="_blank">Click this link</a> to see a simulation of current flowing through a simple circuit. This simulation requires Java to run.</p>
<hr />
<p><a id="franklin" name="franklin"></a>*Benjamin Franklin originally wrote that electricity flows from the positive side of a voltage source to the negative side. However, Franklin had no way of knowing that <a href="http://bit.ly/2nL8ccd" target="_blank">electrons actually flow in the opposite direction</a> – at the atomic level, they come out of the negative side and loop back to the positive side. Because engineers followed Franklin’s lead for hundreds of years before the truth was discovered, we still use the “wrong” convention to this day. Practically speaking this detail doesn’t matter, and as long as everyone uses the same convention, we can all build circuits that work just fine.</p>
</div>
<div class="section tutorial-page">
<h2 id="short-and-open-circuits">Short and Open Circuits</h2>
<h3>What is a “Load”?</h3>
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<p>The reason we want to build circuits is to make electricity do useful things for us. The way we do that is by putting things in the circuit that use the current flow to <a href="http://bit.ly/2fmjZIw" itemprop="url" target="_bank" title="Arduino Street Traffic Light - Breadboard Edition"><strong>light</strong></a> up, make noise, run programs, etc.</p>
<p>These things are called <strong>loads</strong>, because they “load down” the power supply, just like you’re “loaded down” when you’re carrying something. The same way you could be loaded down with too much weight, it’s possible to load down a power supply too much, which will slow down the current flow. But unlike you, it’s also possible to load down a circuit too little – this may let too much current flow (imagine running too fast if you weren’t carrying any weight), which can burn out your parts or even the power supply.</p>
<p>You’ll learn all about voltage, current, and loads in the next tutorial: <a href="http://bit.ly/2nLgalw" target="_blank">Voltage, Current, Resistance, and Ohm’s Law</a>. But for now, let’s learn about two special cases of circuit: <strong>short circuit</strong>, and <strong>open circuit</strong>. Knowing about these will help tremendously when you’re troubleshooting your own circuits.</p>
<h3>Short Circuit<a id="short" name="short"></a></h3>
<p>DON’T DO THIS, but if you connect a wire directly from the positive to the negative side of a power supply, you’ll create what is called a <strong>short circuit</strong>. This is a very bad idea.</p>
<p><a href="http://bit.ly/2pf3UXA" target="_blank"><img alt="alt text" src="http://bit.ly/2pf3UXA" /></a></p>
<p>This seems like the best possible circuit, so why is it a bad idea? Remember that electrical current wants to flow from a higher voltage to a lower voltage, and if you put a load into the current, you can do something useful like light up an LED.</p>
<p>If you DO have a load in the current, the current flow through your circuit will be limited to that which your device consumes, which is usually a very small amount. However, if you DON’T put anything in to restrict the current flow, there won’t be anything to slow down the current, and it will try to be infinite!</p>
<p>Your power supply can’t provide infinite current, but it will provide as much as it can, which may be a lot. This could cause your wire to burn up, damage the power supply, drain your battery, or other exciting things. Most of the time your power supply will have some sort of safety mechanism built into it to limit the maximum current in the event of a short circuit, but not always. This is the reason all homes and buildings have <a href="http://bit.ly/2nLeEA2" target="_blank">circuit breakers</a>, to prevent fires from starting in the event of a short circuit somewhere in the wiring.</p>
<p>A closely related problem is accidentally letting too much current flow through part of your circuit, causing a part to burn up. This isn’t quite a short circuit, but it’s close. This most often happens when you use the incorrect <strong>resistor</strong> value, which lets too much current flow through another component such as an LED.</p>
<p><a href="http://bit.ly/2pcZ8wI" target="_blank"><img alt="alt text" src="http://bit.ly/2pcZ8wI" /></a></p>
<p>The bottom line: <strong>if you notice that things are suddenly becoming hot or a part suddenly burns out, immediately turn off the power and look for possible short circuits.</strong><a id="open" name="open"></a></p>
<h3>Open Circuit</h3>
<p>The opposite of a short circuit is an <strong>open circuit</strong>. This is a circuit where the loop isn’t fully connected (and therefore this isn’t really a circuit at all).</p>
<p><a href="http://bit.ly/2odOvVV" target="_blank"><img alt="alt text" src="http://bit.ly/2odOvVV" /></a></p>
<p>Unlike the short circuit above, nothing will get hurt by this “circuit”, but your circuit won’t work either. If you’re new at circuits, it can often be hard to find where the break is, especially if you’re using <a href="http://bit.ly/2kamGiX" target="_blank">breadboards</a> where all the conductors are hidden.</p>
<p><strong>If your circuit doesn’t work, the most likely cause is an open circuit.</strong> This is usually due to a broken connection or a loose wire. (Short circuits can steal all the power from the rest of your circuit, so be sure to look for those as well.)</p>
<p><strong>TIP:</strong> if you can’t easily find where your circuit is open, a <strong><a href="http://bit.ly/2odPSnn" target="_blank">multimeter</a></strong> can be very useful tool. If you set it to measure volts, you can use it to check the voltage at various points in your powered circuit, and eventually find the point where voltage isn’t getting through.</p>
</div>
<div class="section tutorial-page">
<h2 id="re">Resources and Going Further</h2>
<p>You’ve just learned, in its most basic form, what a circuit is. As you keep learning, you’ll encounter more complex circuits that have multiple loops and many more electronic components. But ALL circuits, no matter how complex, will follow the same rules as the basic one-loop circuit you just learned about.</p>
<p>Your journey into electronics is just beginning, here are some suggested next topics to explore:</p>
<ul>
<li><a href="http://bit.ly/2kamGiX" target="_blank">Breadboards</a> are useful tools that let you quickly build temporary circuits using jumper wires. We use them all the time. You also may want to master <a href="http://bit.ly/2pcOoON" target="_blank">working with wire</a> to help you build your circuits.</li>
<li>A <a href="http://bit.ly/2odPSnn" target="_blank">multimeter</a> lets you measure voltage, current, and resistance and is a great help when trying to troubleshoot malfunctioning circuits.</li>
<li>Circuits come in all different sizes, shapes, and configurations. Check out the <a href="http://bit.ly/2odRSfA" target="_blank">series vs parallel circuits</a> tutorial to see circuits taken to the next level.</li>
</ul>
<p>Here are some tutorials on the most common components you’ll use when building circuits.</p>
<ul>
<li>A great way to learn about circuits is to start making some. Our <a href="http://bit.ly/1WBzUB8" target="_blank">LED tutorial</a> will show how to light up one or many LEDs.</li>
<li><a href="http://bit.ly/2odGTTm" target="_blank">Resistors</a> are one of the most widely used components in circuits.</li>
<li><a href="http://bit.ly/2pd3ZOi" target="_blank">Capacitors</a> are also found in most circuits. As are <a href="http://bit.ly/2odxMC6" target="_blank">Diodes</a>.</li>
</ul>
</div>
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What is a Circuit?

Overview Welcome to circuits 101! One of the first things you’ll encounter when learning about electronics is the concept of a circuit . T...

SIK Experiment Guide for Arduino – V3.3

Wall's Corners

Link Author

Title: SIK Experiment Guide for Arduino – V3.3
Author: Wall's Corners
Rating 5 of 5 Des:

Introduction: Hardware The SparkFun Inventor’s Kit is your map for navigating the waters of beginning embedded electronics. This kit contai...

<h2>Introduction: Hardware</h2>
<p>The SparkFun Inventor’s Kit is your map for navigating the waters of beginning embedded electronics. This kit contains all the information and parts you will need to create 16 circuits that cover the basics of programming and hardware interactions. At the center of this kit is one core philosophy – that anyone can (and should) experiment with electronics. When you’re done with this guide, you’ll have the know-how to start creating your own projects and experiments.</p>
<p>This guide is also available as a downloadable PDF, if you prefer.</p>
<p><a class="btn btn-default" href="http://bit.ly/2pcWRS9" target="_blank">SIK GUIDE DOWNLOAD</a></p>
<h3>SparkFun Inventor’s Kit – V3.3</h3>
<p>You should have one of the two following versions of the SIK. If you need a overview of the parts included in your kit, please click on the product link below.</p>
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<h3 class="title"><a href="http://bit.ly/2odDrbq" target="_blank">SparkFun Inventor’s Kit – V3.3</a></h3>
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<h3 class="title"><a href="http://bit.ly/2pcNguC" target="_blank">SparkFun Inventor’s Kit (for Arduino Uno) – V3.3</a></h3>
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</div>
</div>
<p>The primary difference between the two kits is the microcontroller included in the kit. The SparkFun Inventor’s Kit includes a <a href="http://bit.ly/2odTHci" target="_blank">SparkFun RedBoard</a>, while the SparkFun Inventor’s Kit for <a href="http://bit.ly/26nCm2w" itemprop="url" target="_bank" title="Arduino"><strong>Arduino</strong></a> Uno includes an <a href="http://bit.ly/2odTHsO" target="_blank">Arduino Uno R3</a>. At the heart of each is the ATmega328p microcontroller, giving both the same functionality underneath the hood. Both development boards are capable of taking inputs (such as the push of a button or a reading from a <a href="http://bit.ly/1WMCgNC" itemprop="url" target="_bank" title="Making an LED Light up with Raspberry Pi Before Making It Blink"><strong>light</strong></a> sensor) and interpreting that information to control various outputs (like a blinking LED light or an electric motor). And much, much more!</p>
<div class="alert alert-warning"><b>Note:</b> The <a href="http://bit.ly/1XReOgM" itemprop="url" target="_bank" title="Arduino"><strong>Arduino</strong></a> Uno version of the kit <b>does not</b> include a <a href="http://bit.ly/2pcJFwI" target="_blank">carrying case</a> or printed copy of this manual to decrease weight and cost for international shipping.</div>
<div class="alert alert-info"><b>Note:</b> You can complete all 16 experiments in this guide with either kit.</div>
<p>If you need more information to determine which microcontroller is right for you, please check out the following tutorials.</p>
<div class="row">
<div class="col-md-6">
<div class="tile-wrap">
<div class="tile tutorial-tile has_addl_actions big grid" data-id="129">
<div class="thumb-wrap"></div>
<h3 class="title">RedBoard Hookup Guide</h3>
<h4 class="metaline">JANUARY 7, 2014</h4>
<div class="description">How to get your RedBoard up-and-blinking!</div>
</div>
</div>
<div class="clearfix"></div>
</div>
<div class="col-md-6">
<div class="tile-wrap">
<div class="tile tutorial-tile has_addl_actions big grid" data-id="50">
<div class="thumb-wrap"></div>
<h3 class="title">What is an Arduino?</h3>
<h4 class="metaline">FEBRUARY 26, 2013</h4>
<div class="description">What is this ‘Arduino’ thing anyway?</div>
</div>
</div>
<div class="clearfix"></div>
</div>
</div>
<h3>Open Source!</h3>
<p>At SparkFun, our engineers and educators have been improving this kit and coming up with new experiments for a long time now. We would like to give attribution to <a href="http://oomlout.com/" target="_blank">Oomlout</a>, since we originally started working off their Arduino Kit material many years ago. The Oomlut version is licensed under the <a href="http://bit.ly/2odWbYl" target="_blank">Creative Commons Attribution Share-Alike 3.0 Unported License</a>.</p>
<p>SparkFun’s version 3.3 is licensed under the <a href="http://bit.ly/2pd2IHc" target="_blank">Creative Commons Attribution Share-Alike International License</a>.</p>
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SIK Experiment Guide for Arduino – V3.3

Introduction: Hardware The SparkFun Inventor’s Kit is your map for navigating the waters of beginning embedded electronics. This kit contai...

Building a Safe Cracking Robot

Wall's Corners

Link Author

Title: Building a Safe Cracking Robot
Author: Wall's Corners
Rating 5 of 5 Des:

Introduction My wife is amazing; for Christmas this year, she gave me a fire safe she purchased off of Craigslist. It was super cheap beca...

<div class="section tutorial-page">
<h2 id="introduction">Introduction</h2>
<p>My wife is amazing; for Christmas this year, she gave me a fire safe she purchased off of Craigslist. It was super cheap because the seller didn’t have the combination. Best present ever.</p>
<p>Step 1) Get a safe that hasn’t been opened.</p>
<p>Step 2) Deploy robot army.</p>
<p><a href="http://bit.ly/2pcMWfb" target="_blank"><img alt="Cracked combination safe" src="http://bit.ly/2odDqEo" /></a></p>
<p><em>Combination 24/0/66</em></p>
<p>We were able to crack our Craigslist safe in 40 minutes and 42 seconds! You can <a href="https://www.youtube.com/watch?v=AsVSEHv2N4M" target="_blank">re-watch the live cracking</a>, if you’d like. The magic moment occurs at <a href="https://www.youtube.com/watch?v=AsVSEHv2N4M&t=45m10s" target="_blank">45:20</a>, but start around <a href="https://www.youtube.com/watch?v=AsVSEHv2N4M&t=44m30s" target="_blank">44:30</a> to get the full scope of what’s happening.</p>
<p><a href="http://bit.ly/2pcRX7F" target="_blank"><img alt="SparkFun Safe Cracker" src="http://bit.ly/2odOBg8" /></a></p>
<p>Brute force robots for combination safes are not new. They are sometimes called an “Autodialer”. And, the robots that use audio feedback are sometimes called “SoftDrill”. The SparkFun Safe Cracker is a variant of the Autodialer. Instead of attempting every combination in the solution domain (called brute forcing), we use some tricks to reduce the domain and shortcuts to speed up the testing process. In addition, the SparkFun Safe Cracker is cheap (~$200), magnetically adheres to the safe, and is non-destructive; when we’re done you’ll never know we were there.</p>
</div>
<div class="section tutorial-page">
<h2 id="isnt-this-a-bad-idea">Isn’t this a bad idea?</h2>
<p>We believe knowledge and education is the best protection against fear and tyranny. The SparkFun Safe Cracker is designed to open very low security combination fire safes. There are high-end, secure, expensive combination safes available that have the ability to detect and thwart this type of dialer attack. Or, you could use a keypad safe.</p>
</div>
<div class="section tutorial-page">
<h2 id="build-your-own">Build Your Own!</h2>
<p>We’ve documented and shared all the lessons we’ve learned in hopes that you can possibly build your own. You’ll need a 3D printer, soldering iron, and the ability to write code and modify 3D files to fit the type of safe you’re trying to open.</p>
<p>This is a complex build so here’s a list of documents:</p>
<ul>
<li><a href="http://bit.ly/2pd4YOt" target="_blank">Eagle files for the PCB</a></li>
<li><a href="http://bit.ly/2odF5to" target="_blank">Schematic</a></li>
<li><a href="http://bit.ly/2pd3TWW" target="_blank">Arduino based firmware</a></li>
<li><a href="http://bit.ly/2odPLrX" target="_blank">3D files</a> for the dial coupler and handle puller</li>
<li><a href="http://bit.ly/2odPLrX" target="_blank">3D model</a> of the safe</li>
<li>SparkFun <a href="http://bit.ly/2pd8bOd" target="_blank">parts list</a></li>
<li>Additional Parts:
<ul>
<li>$40 <a href="http://bit.ly/2odTsxW" target="_blank">DC gearhead motor</a></li>
<li><a href="http://bit.ly/2pcLGZy" target="_blank">Power supply connector</a></li>
<li>Three <a href="http://bit.ly/2odRMoe" target="_blank">BCA6 magnets</a> worked very well</li>
<li><a href="http://bit.ly/2pd4YOt" target="_blank">Custom PCB</a> (order from your favorite fab house)</li>
</ul>
</li>
</ul>
<p>All in all, we’ve spent about $200 on our safe cracker, which is a fraction of what professional devices cost.</p>
<p>For the latest files, check out the <a href="http://bit.ly/2pcToTP" target="_blank">repo here</a>.</p>
</div>
<div class="section tutorial-page">
<h2 id="dial-and-coupler">Dial and Coupler</h2>
<p><a href="http://bit.ly/2odTeql" target="_blank"><img alt="Dial coupler" src="http://bit.ly/2pcHeKE" /></a></p>
<p>The dial on our safe was modeled, and a coupler was 3D printed. This was attached to our motor with a <a href="http://bit.ly/2odSnWR" target="_blank">6mm clamping hub</a>. We found that hubs with set screws would loosen quickly and wreak havoc with our control algorithms. Use a clamping hub!</p>
<p>We used a motor from <a href="http://bit.ly/2odTsxW" target="_blank">Pololu</a> for $40 that has the following specs</p>
<ul>
<li>12V motor</li>
<li>350mA Free Run</li>
<li>5A stall</li>
<li>80RPM</li>
<li>8400 counts per revolution</li>
</ul>
<p>The counts per revolution was most important. We wanted a lower cost motor that had LOTS of resolution to measure the internal indents in disc C (covered in a later section). 12V was good because we had used a similar power supply and display for our <a href="https://www.youtube.com/watch?v=uC9CkhJiIaQ" target="_blank">Speed Trap</a> project.</p>
<p>The encoder uses two interrupt pins connected to the interrupt pins on the Arduino. Once we zero the dial, the step count is used to determine on which digit the dial is.</p>
<p>Quick example:</p>
<p>100 / 8400 = 84 ticks per digit 3226 steps = dial number 34</p>
<p>It’s worth noting that the tolerances of the coupler to dial and the rigidness of the hub and motor mounts are important. Any slack in the system will cause problems later.</p>
<p><strong>Home Calibration</strong></p>
<p>We tried a few different methods to calibrate the dial. Originally, we tried a reed switch with a magnet built into the 3D printed coupler. This was a horrible idea: reed switches detect relative proximity but are bad at detecting exact location. I had a few instances where the reed switch would open when the magnet was directly under the switch and close again when the magnet moved a few millimeters away. Don’t use a reed switch.</p>
<p><a href="http://bit.ly/2pd13RV" target="_blank"><img alt="Photo gate above dial coupler" src="http://bit.ly/2odTupy" /></a></p>
<p>We ended up using a photogate with a small flag designed into the coupler. It’s straightforward to detect when the gate is broken by the flag. When the robot is magnetically attached to the safe, the dial is in an unknown position. There’s an offset variable that can be set so that when you say, “go home”, the robot figures out that the flag is at dial position 43, and it needs to travel 57 more to arrive at home.</p>
<p>Once calibrated (and after lots of code revisions), we found that the control of the dial was very reliable. During cracking, we recalibrate every time we adjust disc A just to be safe.</p>
</div>
<div class="section tutorial-page">
<h2 id="handle-puller">Handle Puller</h2>
<p><a href="http://bit.ly/2pcWRBD" target="_blank"><img alt="Handle puller with nautilus gear" src="http://bit.ly/2odSpOt" /></a></p>
<p>We tried a few different methods to non-permanently attach to the handle. First, we had to model the safe handle then design and print a connector. It’s basically a shroud with an idler pin to rotate around with a 40 lbs cord attached to the end of the shroud. A small spring is used to bring the shroud and handle back to return position when the servo returns to the rest position. You don’t want to let the handle rest under its own weight or it might fall onto the dials and get caught in the indents on disc C.</p>
<p>We really wanted to use an off-the-shelf servo for cost and ease of use. A basic <a href="http://bit.ly/2pcZGm0" target="_blank">83 oz-in servo</a> worked ok, but it didn’t have enough throw to guarantee that it could pull the handle down far enough to the open position. Rob had the breakthrough: using a nautilus design we can apply increasing torque to the handle as the servo head pulls on the string. It works extremely well.</p>
<p>Additionally, we modified our servo to give us analog feedback (there are good tutorials <a href="http://bit.ly/2odOpxx" target="_blank">1</a> and <a href="http://bit.ly/2pcOnud" target="_blank">2</a> on how to do this). Analog feedback is important; when you tell the servo head to go to position 175, did it actually get there? Doing an <code>analogRead()</code> lets us know if we’ve arrived.</p>
<p>To detect if the safe is opened, we tell the servo to pull on the handle by going to PWM value 80. If we’ve dialed in the right combination, the the servo is free to fully pull down and move to this PWM value. The analog value when the handle is pulled down is approximately 273. If the servo gets blocked (wrong combination) the servo’s analog feedback will be less than 250. The <code>tryHandle()</code> function does all this for us and returns true if the analog feedback goes above 260. The PWM and analog values will be different for each robot setup, so we’ve written a few functions to manually adjust the servo to get the opening conditions.</p>
</div>
<div class="section tutorial-page">
<h2 id="brains">Brains</h2>
<p><a href="http://bit.ly/2odDqUU" target="_blank"><img alt="Robot cracking Arduino shield" src="http://bit.ly/2pcS1nV" /></a></p>
<p>We used the trusty <a href="http://bit.ly/2odTHci" target="_blank">RedBoard</a> to control everything. The Safe Cracker shield is a simple two layer PCB.</p>
<p>Bits on the shield:</p>
<ul>
<li>The 12V/5V power supply provides the power to the current sensor and motor controller.</li>
<li>Current sensor: We originally planned on using the current sensor to detect motor stall when the edge of an indent in disc C was hit. It turned out being much faster and more accurate to detect the indent edge with the encoder; if the encoder had not changed in 5 milliseconds then the motor was stopped and we had hit the edge of the indent.</li>
<li>Motor controller: 15A was more than enough to handle the motor’s 5A stall current</li>
<li>Servo connector with feedback</li>
<li>Photogate connector</li>
<li>Display connector for the <a href="http://bit.ly/2pcOpCl" target="_blank">large 6.5” 7-segment display</a> (needed only for our live stream event)</li>
<li>Buzzer to announce when we’ve cracked it</li>
<li>‘GO’ button connector if we wanted to make the apparatus headless</li>
</ul>
<p><a href="http://bit.ly/2odF5to" target="_blank"><img alt="Safe Cracker Schematic" src="http://bit.ly/2odSyl7" /></a></p>
<p><strong>Code</strong></p>
<p>You can find the firmware <a href="http://bit.ly/2pcToTP" target="_blank">here</a>. While the code to control the SparkFun Safe Cracker got a bit large, it’s fairly straight forward. In essence, we go to a given dial location, pull on the handle, see if the handle moved far enough that we’re open, repeat. Additionally, we created functions to allow us to measure, as precisely as possible, the widths of the indents on disc C.</p>
</div>
<div class="section tutorial-page">
<h2 id="frame">Frame</h2>
<p><a href="http://bit.ly/2pcPsCa" target="_blank"><img alt="alt text" src="http://bit.ly/2odTJ3U" /></a></p>
<p>To build an apparatus that could be easily and quickly attached to our safe, we first modeled the safe in 3D.</p>
<p><a href="http://bit.ly/2pcNge6" target="_blank"><img alt="alt text" src="http://bit.ly/2odF5JU" /></a></p>
<p>The frame is built with <a href="http://bit.ly/2pcUjDw" target="_blank">Actobotics parts</a> with magnets hot glued to the three feet. The magnets provide excellent adhesive strength while still being able to attach and detach the robot.</p>
<p><a href="http://bit.ly/2odOBwE" target="_blank"><img alt="alt text" src="http://bit.ly/2pcRWAL" /></a></p>
<p>The handle cover, coupler, base plate to hold the electronics, and nautilus gear were printed on our trusty <a href="http://bit.ly/2odUMRx" target="_blank">Taz 6</a> printer.</p>
<p><a href="http://bit.ly/2pd2Giy" target="_blank"><img alt="alt text" src="http://bit.ly/2pd2Giy" /></a></p>
<p>This frame is specifically designed for our model safe, but the variety of <a href="http://bit.ly/2pcUjDw" target="_blank">Actobotics parts</a> and the ability to print custom parts means the SparkFun Safe Cracker could be modified to fit any particular model of safe.</p>
</div>
<div class="section tutorial-page">
<h2 id="how-combination-safes-work">How Combination Safes Work</h2>
<p>In order to understand some of the shortcuts we took, you’ll need to know how the discs inside a combination lock operate. Here’s a quick primer!</p>
<p><a href="http://bit.ly/2odH54Z" target="_blank"><img alt="Combination lock" src="http://bit.ly/2pcLF7W" /></a></p>
<p>If you’ve ever used a combination padlock you know the basics:</p>
<ul>
<li>Spin the dial a few times to reset everything</li>
<li>Turn the dial in a certain direction until you get to the first combination number. Let’s call that AA.</li>
<li>Turn the dial in the opposite direction one full turn, then continue until you get to the second number – BB</li>
<li>Turn the dial in the opposite direction until get you get to the third number – CC</li>
<li>Pull on the handle to open lock</li>
</ul>
<p>The most common padlocks have a dial from 0 to 39 with a combination AA/BB/CC. Combination safes work exactly the same but with larger dials, usually 0 to 99. Some safes have additional combination numbers (for example: AA/BB/CC/DD/EE), but the general home-store fire safe is 3 numbers.</p>
<p>But, how do the internals of a combination safe actually work?</p>
<p>Here’s a video to show you the basics:</p>
<div class="flex-video-wrap clearfix">
<div class="flex-video widescreen img"><iframe src="https://www.youtube.com/embed/5HtdLiOqetc/?autohide=1&border=0&wmode=opaque&enablejsapi=1" width="560" height="315" frameborder="0" allowfullscreen="allowfullscreen"></iframe></div>
</div>
<p>There are three discs, let’s call them discs A, B and C. Each disc has a notch in it called a gate. When you pull down on the handle, a rod (sometimes called the fence) hits the three dials. If the three gates are lined up correctly, the rod slides into the notches. This allows the handle to travel far enough to disengage the lock on the door, and the safe can be opened.</p>
<p>Turning the dial on the outside of the lock directly controls disc C (sometimes called the drive wheel). Twist it clockwise (CW) or counterclockwise (CCW), and you directly manipulating the C disc. But, how do you move the other two discs?</p>
<p><a href="http://bit.ly/2odH6WB" target="_blank"><img alt="Combination lock discs with raised tabs" src="http://bit.ly/2pcUzlW" /></a></p>
<p><em>Discs from the inside of a fire safe</em></p>
<p>Each disc has a raised plastic tab. When the discs are next to each other, a disc can move freely for about 350 degrees until its raised tab hits the tab on the next disc and begins moving it.</p>
<p>If you turn disc C a full turn, the tab hits the tab on disc B and begins to turn it. Similarly, disc B has a tab on the opposite side of the disc that will hit disc A’s tab. Turn the dial far enough, and C will pick up and start turning B, which will then pick up and start turning A.</p>
<p>Clear as mud? Check out <a href="http://bit.ly/2odPLIt" target="_blank">Woodgears</a>. They have a great breakdown of the pieces and a video showing how the discs are manipulated.</p>
<div class="flex-video-wrap clearfix">
<div class="flex-video widescreen img"><iframe src="https://www.youtube.com/embed/CZ8WRDVgKrk/?autohide=1&border=0&wmode=opaque&enablejsapi=1" width="560" height="315" frameborder="0" allowfullscreen="allowfullscreen"></iframe></div>
</div>
</div>
<div class="section tutorial-page">
<h2 id="the-problem-domain">The Problem Domain</h2>
<p>The dial on our SafeSentry safe is 100 digits. If there are three discs, our domain is 100 * 100 * 100 or 1 million possible combinations. Testing on a safe at the store, we found it took about ten seconds on average to reset the dial then dial in the three solution numbers. So, worst case, it will take 115.74 days to try every combination. And 10 seconds is pretty fast; humans get tired and less precise over time. Luckily, there are some tricks we can do.</p>
<p>On some high-end, secure safes, if you turn the dial to 81.5 and the combination is 81, it won’t open. With these lower cost home-brand safes the manufacturing tolerances are much larger. On the safe we tried at the store if one of the numbers in the combination is 53 then 52 and 54 will also work.</p>
<div class="flex-video-wrap clearfix">
<div class="flex-video widescreen img"><iframe src="https://www.youtube.com/embed/5HtdLiOqetc/?autohide=1&border=0&wmode=opaque&enablejsapi=1" width="560" height="315" frameborder="0" allowfullscreen="allowfullscreen"></iframe></div>
</div>
<p>The first part of this video will show you the tolerances on the combination dial are +/-1 digit.</p>
<p>This quickly reduces the domain to 33 * 33 * 33 = 35,937 combinations. That’s still over 4 days of trying.</p>
<p>One of the ways to pick a safe is by feel. Manufacturers know this, so, to prevent it, the last disc (we’ll call it disc C) has a series of indents. If you press down on the handle and spin the dial, the thing trying to push down into the notches on the dials, called the <em>fence</em>, will fall into these false indents and lock up the dials. Bummer. But, you quickly figure out that there are 12 indents. And, one of these ‘indents’ must be the correct slot when you dial in the combination.</p>
<p>The problem domain is now 33 * 33 * 12 = 13,068 combinations or 1.51 days. About the speed of paint drying.</p>
<p>The locations of these indents are found easily by <a href="http://bit.ly/2dBP8pn" itemprop="url" target="_bank" title="hand"><strong>hand</strong></a>. 100 / 24 (12 indents, 12 raised bits) = 4.17 dial positions per indent. It’s not super clean, but, if an indent ranges 21 to 25.2, then it’s safe to go to 24 to ‘test’ that indent to see if it’s actually a solution slot.</p>
<p>On our safe we located the indents as follows:</p>
<div class="copy-code" data-code-id="code-0"><button class="btn btn-default btn-sm">COPY CODE</button></div>
<pre>
<code>case 0: return (99); //98 to 1 on the wheel
case 1: return (8); //6-9
case 2: return (16); //14-17
case 3: return (24); //23-26
case 4: return (33); //31-34
case 5: return (41); //39-42
case 6: return (50); //48-51
case 7: return (58); //56-59
case 8: return (66); //64-67
case 9: return (74); //73-76
case 10: return (83); //81-84
case 11: return (91); //90-93
</code>
</pre>
<p>Here’s what disc C looks like:</p>
<p><a href="http://bit.ly/2odUOJ9" target="_blank"><img alt="Disc from combination safe showing indents" src="http://bit.ly/2pd08kf" /></a></p>
<p><em>11 false gates, 1 solution gate</em></p>
<p>Disc C has the following dimensions:</p>
<ul>
<li>Outer diameter: 2.815” (55.5mm)</li>
<li>Width of solution slot: 0.239”</li>
<li>Width of 11 indents: 0.249” +/- 0.002”</li>
</ul>
<p>C = 2πr, so our circumference is 17.69”. Our motor has an 8400 tick encoder. Each encoder tick is therefore approximately 0.0021”. So, we’re looking for a difference of about 5 ticks. Eeek! That’s not many.</p>
<p>If we can externally measure the widths of the various slots, we may be able to discern which slot is the skinniest and therefor the solution slot. This would reduce our combination domain to 33 * 33 * 1 = 1,089 and a crack time of 3 hours, worst case.</p>
<p>The function <code>measureDiscC()</code> is designed to take a series of readings and add them together. The motor is a gear head motor and has a tremendous amount of torque, so, if there is any flexing or give in your apparatus, this will show up in the readings as noise. However, if we take many simultaneous readings, any flexion should be replicated to all indent measurements allowing the skinny slot to bubble to the top.</p>
<p><strong>Set screws will not work.</strong> A hub that uses a set screw to connect to the D-shaft on a motor will not survive the constant torture of indent measuring. Once we switched to a 6mm clamping hub we had much less noisy readings.</p>
<p>Here’s the output from five <code>measureDiscC()</code> tests on our Craigslist safe with no combination:</p>
<div class="copy-code" data-code-id="code-1"><button class="btn btn-default btn-sm">COPY CODE</button></div>
<pre>
<code>Measuring complete
Smallest to largest width [Width]/[Depth]
Indent 8: [1911] / [1130]
Indent 1: [1925] / [1122]
Indent 3: [1953] / [1091]
Indent 0: [1955] / [1099]
Indent 11: [1966] / [1105]
Indent 2: [1992] / [1100]
Indent 9: [1994] / [1126]
Indent 7: [2011] / [1098]
Indent 10: [2036] / [1096]
Indent 4: [2077] / [1109]
Indent 5: [2083] / [1100]
Indent 6: [2114] / [1096]
</code>
</pre>
<p>Indent 8 bubbles to the top on almost all the tests we have run on our safe. We also output the depth measurements (how far does the handle go down), but I am much more suspicious of these readings.</p>
<p>We can’t be sure the smallest indent is the solution ident, or if we’ve even measured the indents correctly so the Safe Cracker firmware allows the user to control which indents are to be tested. Turn them all on, turn on 5, or turn on only one, it’s up to you. We recommend trying to crack your safe with the smallest four indents. If you fail to open the safe then turn these 4 off, turn the other 8 indents to true, and run again.</p>
<p>We are down to 33 * 33 * 4 = 4,356 or a little over 12 hours. Still not great. What other tricks can we do?</p>
<p><strong>Quick Note:</strong> When we live streamed the safe cracking, we were conservative and selected four indents to try. The winning indent was indent 8, the skinniest indent. So with two data points, I’d say this vulnerability has potential. You can <a href="https://www.youtube.com/watch?v=AsVSEHv2N4M" target="_blank">re-watch the stream</a> if you’d like. The magic moment occurs at <a href="https://www.youtube.com/watch?v=AsVSEHv2N4M&t=45m10s" target="_blank">45:20</a>, but start around <a href="https://www.youtube.com/watch?v=AsVSEHv2N4M&t=44m30s" target="_blank">44:30</a> to get the full scope.</p>
</div>
<div class="section tutorial-page">
<h2 id="set-testing">Set Testing</h2>
<p>If each three combination attempt takes 10 seconds, what can we reduce the time per attempt? We can dial faster of course. But, we can also get sneaky with how we adjust the dials. It takes me 10 seconds because I can’t back up a digit without fouling where the discs are sitting. The robot can be much more precise.</p>
<p>To crack our safe, we set discs A and B, test the indents on disc C, move backwards to adjust disc B, then test the indents again. We call this <strong>set testing</strong> as opposed to reset testing (where a full reset is done between tests).</p>
<p>The robot attempts combinations this way:</p>
<ol>
<li>Reset dials by spinning CCW, past zero, continue until we arrive at zero.</li>
<li>Continue CCW to 3. This sets disc A to 3.</li>
<li>Rotate CW to 3. Continue CW to 0. Disc B is now set to 0.</li>
<li>Rotate CCW to the first allowed indent; ours is 8. Disc C is now set to 8.</li>
<li>Try the handle. Failed? Continue…</li>
<li>Rotate CCW to next allowed indent; ours is 24.</li>
<li>Try the handle. Failed? Continue…</li>
<li>Rotate CCW to next allowed indent; ours is 66.</li>
<li>Try the handle. Failed? Continue…</li>
<li>Rotate CCW to next allowed indent; ours is 74.</li>
<li>Try the handle. Failed? Continue…</li>
<li>Rotate CW to 6. Disc B is now set to 6.</li>
<li>Loop to step 4.</li>
<li>Rotate CW to 9. Disc B is now set to 9.</li>
<li>etc…</li>
</ol>
<div class="flex-video-wrap clearfix">
<div class="flex-video widescreen img"><iframe src="https://www.youtube.com/embed/5HtdLiOqetc/?autohide=1&border=0&wmode=opaque&enablejsapi=1" width="560" height="315" frameborder="0" allowfullscreen="allowfullscreen"></iframe></div>
</div>
<p>Here’s a video of set testing, done by <a href="http://bit.ly/1PhUuAm" itemprop="url" target="_bank" title="ANIMATRONIC ROBOTIC HAND"><strong>hand</strong></a>, to demonstrate what we’re talking about. Jump to 0:41 to see Set Testing in action.</p>
<p>Using this method we can test the AA/BB/xx combinations (set Disc B to 8 then test four indents) in approximately 8 seconds vs. 40 seconds (4 test @ 10s per test). Much faster!</p>
<p>Originally: 33 * 33 * 40s = 12.1 hours</p>
<p>Using Set Testing: 33 * 33 * 8.3s = 2.5 hours worst case!</p>
<p>And, if we’re confident in our single indent the test time comes down to approximately 4 seconds per test (it’s not linear because of the time to move disc B).</p>
<p>Set Testing with one indent: 33 * 33 * 4s = 1.2 hours.</p>
</div>
<div class="section tutorial-page">
<h2 id="additional-resources">Additional Resources</h2>
<p>We hope you enjoyed reading about our safe cracking robot. If you have any questions or discover any other tricks, please <a href="http://bit.ly/2pd4WWR" target="_blank">let us know</a>!</p>
<p>There are some great videos on how to crack safes, just have a poke around. Eric Schmiedl’s talk <a href="https://www.youtube.com/watch?v=4_lkYQ88kv0" target="_blank">Safecracking Without a Trace</a> at DEFCON 14 was especially good.</p>
</div>
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Building a Safe Cracking Robot

Introduction My wife is amazing; for Christmas this year, she gave me a fire safe she purchased off of Craigslist. It was super cheap beca...

What is a Circuit?

SIK Experiment Guide for Arduino – V3.3

Building a Safe Cracking Robot

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